1. Field of the Invention
This invention relates to a superconductive substance whose electric resistance disappears at a low temperature. More particularly, the invention relates to a so-called high-temperature superconductor which can be made superconductive by liquid nitrogen coolant.
2. Description of the Prior Art
With conventional superconductors, cooling by costly liquid helium (whose boiling point is 4.2 K) at very low temperatures is indispensable. Practical use of the superconductor has been restricted only to very special cases due to the high cooling cost and the limited availability of helium resources in certain countries.
The superconductive substance which has the highest critical temperature so far has been confirmed to be Nb.sub.3 Ge. Its critical temperature (i.e. transformation-starting temperature) Tc is 23.6.degree. K. which is not high enough to be achieved by cooling with liquid nitrogen (whose boiling point is 77.degree. K.).
It has been reported that superconductors made of oxides of La-(Ba, Sr)-Cu system have a transformation temperature in a range of 25.degree.-45.degree. K. However, the temperature at which the electric resistance of the above oxides completely disappears is about 25.degree. K. With the above oxides, that temperature at which they start to become superconductive is lower than the nitrogen boiling point (i.e., 77.degree. K.), and the highest of such superconductivity-starting temperatures reported in Japan is 54.degree. K. Thus, such substance which starts to become superconductive at a temperature above the nitrogen boiling point of 77 K has not been developed yet.
Heretofore, the coolant for producing superconductivity has been limited to either liquid helium at very low temperatures or liquid hydrogen (whose boiling point is 20 K). Helium is expensive, and liquid hydrogen is dangerous to handle. Accordingly, there has been a need for such new superconductive substance of which critical temperature for superconductivity is so high that the material can be cooled by liquid nitrogen. Liquid nitrogen (whose boiling point under atmospheric pressure is 77 K) as a coolant is much cheaper than liquid helium and safer than liquid hydrogen.
Generally speaking, the following conditions must be met in order to confirm the superconductivity of a substance.
(a) In addition to the data on electric resistance, the structure of the substance is clearly identified. PA1 (b) The properties of the substance are stable and the results of tests on them are reproducible. PA1 (c) The electric resistance rapidly reduces with temperature reduction over a range of a few degrees from a certain temperature (superconductive transformation-starting temperature). PA1 (d) The substance shows the Meissner effects, namely perfect diamagnetism, that is peculiar to the superconductive phenomenon. PA1 (e) Related physical properties, such as the magnetic susceptibility, the critical current, and the critical magnetic field intensity, of the substance are measured and proved.
No superconductive substance having a high critical temperature suitable for liquid nitrogen coolant while meeting all of the above five conditions has been found yet. Thus, such superconductive substance which is practically usable at a temperature above the nitrogen boiling point has not been developed yet.